In recent years, development of communication environments and digital equipment is creating a huge demand for digital content. However, the majority of movie and music contents having MP3 format, Divx format and the like is being illegally shared across online service providers (OSPs), and thus copyright infringement is increasing more and more.
Moreover, with the advent of Web 2.0 era, users who have been merely consumers of digital content are turning into active producers through user created content (UCC), leading to a rapid increase in the use of existing works such as movies or dramas.
To prevent such copyright infringement, the protection on copyright is strengthening crackdown on the illegal distribution of the digital contents, and is compelling the OSPs to deal with illegal infringement of copyright by technical protection measures. Thus, the OSPs are utilizing a forensic marking technique, which is one of the technical protection measures.
The forensic marking technique is to insert information about a seller, copyright holder, or purchaser into a digital content of a video or audio source, which can extract the previously inserted information even if a user modifies the digital content in other forms for ulterior purposes.
By inserting the information into the digital content, when copyright leakage takes place later, it is possible to track the first circulator of the digital content and place blame on him/her, and it is also possible to raise public awareness on copyright protection. This technology is not only available for a particular device or single authorized device, such as an existing DRM (Digital Rights Management), but also freely available for all devices, and may be modified or processed depending on individual purposes. Thus, much research is actively being done due to the advantage of not restricting the user rights of use.
The process of inserting a forensic mark into multimedia content using this technique includes: decoding the entire of the multimedia content, inserting a forensic mark into the decoded entire multimedia content, and re-encoding the content having the forensic mark inserted therein. Therefore, such a process requires a considerably large amount of computation and time.
Moreover, in an ultra-high speed communication network which is currently used at home, transmission time is short to such an extent that the time taken to transmit multimedia content provided by an OSP to a purchaser is several seconds to several minutes. Since, however, it takes long time to insert a forensic mark into the multimedia content, it is impossible to insert the forensic mark at a point of time when a request from a purchaser is made. Therefore, the multimedia contents currently in service should be prepared with a forensic mark inserted therein in advance before a request from a purchaser was made. Hence, the multimedia content of this type does not have information, such as purchaser ID, transmission point of time or the like.
One of the related arts discloses a technique in which a multimedia content with a watermark inserted therein is prepared in advance before a user's request is made, and upon receiving a user's request, identification information of a user is associated with identification information of the multimedia content with the watermark inserted therein, and then the multimedia content having the watermark associated with the identification information of a user is provided to the user. This technique can satisfy the basic requirements of ultra-high speed services because the multimedia content having the watermark is prepared in advance and, upon a user's request, the multimedia content having the watermark can be transmitted in real time.
In addition, for a popular new content, a large number of purchase requests may be made within a short time. To cope with the large number of purse requests, a space for storing an enormous amount of identical copy versions of the popular content with different watermarks is required. However, if prospect of purchasing demand goes wrong, the storage space may be wasted or the identical copy versions of the popular content may be sold out. Moreover, since a watermark contains no purchaser information but identification information having no meaning, it is necessary to use a matching table to identify original purchasers when tracking illegally shared content later.